As used in this application the term "metal detector" is generally directed to metal detectors of the type used by individuals to locate buried metal objects, particularly valuable metal objects. Such metal detectors are to be contrasted with other types of metal detectors, such as military metal detectors used to locate mines, commercial metal detectors used to locate pipes and other large metallic objects, and security metal detectors used in airports and other areas to locate lethal weapons (such as knives and guns). While some of the principles involved in the present invention may be useful in these other types of metal detectors, it is to be kept in mind that the invention was developed for use with metal detectors used to locate valuable metal objects buried in mineralized ground and finds its main use in such detectors.
In the past two major kinds of valuable metal type metal detectors have been marketed--beat frequency oscillator metal detectors and induction balance metal detectors. The present invention is directed to induction balance type metal detectors, which are sometimes also referred to as transmit-receive (T/R) type metal detectors.
Induction balance metal detectors include transmit and receive coils, plus other related coils (such as feedback and null coils) mounted in a detector head. Preferably the main coils, i.e., the transmit, receive and feedback coils, are coaxially mounted. In some detectors these coils are coplanar, as well as coaxial. In any event, the transmit coil is driven by an oscillator, and the coils are formed, sized and positioned such that the receive coil produces substantially no output signal in the absence of a metallic object (including mineralized ground) disrupting the magnetic field produced by the transmit coil. Metallic objects of both a ferrous and a nonferrous nature disrupt the magnetic field produced by the transmit coil; however, in different manners. In the case of ferrous objects, the magnetic field is concentrated by the ferrous object. In the case of a nonferrous object, eddy currents are produced in the object that, in turn, produce magnetic fields. The eddy current produced magnetic fields dissipate the magnetic field produced by the transmit coil, in the region of the object. In either case, the magnetic field produced by the transmit coil is disrupted in a manner that generates a voltage in the receive coil. If the ground is mineralized, it generates a voltage in the receive coil having some similarity to that produced by a ferrous target object. (While the receive coil voltage is similar in some respects, it is dissimilar in others. For example, a ferrous target object creates a voltage pulse when the detector head passes over the target whereas the ground produced voltage is relatively uniform when compared to a metal object.)
The transmit coils of early type induction balance or T/R type metal detectors were driven by a relatively high frequency oscillator until it was discovered that instrument sensitivity was improved when an oscillation frequency in the audio frequency range was used. Audio frequencies in the few thousand Hz range were found to be the best. Even though this change improved the sensitivity of metal detectors, early metal detectors, even those having target identification capabilities (such as those described in U.S. Pat. Nos. 3,826,973, issued to Norman C. Pflaum for "Electromagnetic Gradiometer" and 3,872,380, issued to Robert F. Gardiner for "Metal Detector Distinguishing Between Different Metals by Using a Bias Circuit Actuated by the Phase Shifts Caused by the Metals"), did not operate satisfactorily in mineralized ground. More specifically, as noted above, mineralized ground disrupts the magnetic field produced by the transmit coil and generates a voltage in the receive coil in a manner similar to a ferrous metallic object. As a result, it is difficult and, in many instances impossible, to locate target metal objects in mineralized ground.
The ground mineralization problem in the valuable object metal detector field remained until a technique for excluding the effect of ground mineralization was developed by George C. Payne. This technique is described in U.S. Pat. No. 4,030,026, entitled "Sampling Metal Detector." While the technique described in U.S. Pat. No. 4,030,026 eliminated the undesirable effect of mineralized ground, only the existence of a potentially desirable target was identified. Information about the ferrous/nonferrous nature of the target, the depth of the target, etc. was not produced. Later metal detector developments provided some of this information. For example, the metal detector described in U.S. Pat. No. 4,128,803, issued to George C. Payne for "Metal Dector System With Ground Effect Rejection," produces an output signal whose polarity indicates the nature of the metal forming the target, i.e., ferrous or nonferrous. A subsequent development, also by Gerorge C. Payne, provides information about the depth of the target, i.e., the distance between the detector head and the buried target.
As will be readily appreciated from the foregoing summary of advances in the metal detector field during the past decade, metal detectors have advanced from crude instruments only usable in nonmineralized soil to sophisticated instruments usable in mineralized soil. However, prior to the present invention, metal detectors have been unable to identify with precision the nature of targets buried in mineralized soil, except as to whether a target is ferrous or nonferrous. More precisely, some prior art metal detectors using the technique described in U.S. Pat. No. 4,128,803 can be adjusted to discriminate against (e.g., reject signals related to) some nonferrous targets as well as ferrous targets located in mineralized ground. However, within the area of detectable targets, the nature (e.g., nickel, dime, quarter, $5.00 gold coin, $10.00 gold coin, etc.) of the target cannot be identified. As a result, users of prior art metal detectors are required to dig up all identified targets in order to determine whether a specific target is valuable or invaluable. Not only is this procedure time consuming, it is also objectionable in many regions. For example, in national historical sites, such as civil war battlefields.